Hi
I find with leaner cuts of meat that it is overcooking that makes it tough. So if the recipe says 40-60 minutes (such as Hungarian Beef Goulash in EDC) I'd check it after 30-40 minutes and try a bit - beef can be eaten rare - so see if you are happy and if not cook a bit more.
Poultry, as you probably know, must be cooked through. With chicken all you can base it on is the colour of meat - so if I was unsure I'd cut a thicker piece (or separate a bit in the middle) and checked it is cooked. Though I imagine the issue is more with red meats and chicken can dry out, but doesn't happen so much I find.
Other suggestions would be to make sure you are cutting meat to the suggested size - if it is smaller it will cook more quickly and if larger would take longer. Cheaper (fattier) cuts of meat will be more tender when stewing or slow cooking. Marinating or pounding meat can also help with tenderising. I actually have a slow cooker, but have retired it now that I have a Thermie. :-)
Finally if reducing the cooking time this might effect flavour of sauce, so taste test that too for best results.
Good luck
Anne